1. Field of the Invention
The present invention relates to an image sensor and a manufacturing method for the same.
2. Discussion of the Related Art
Generally, image sensors refer to a semiconductor device that converts an optical image to an electric signal, and may be classified as charge coupled devices (CCD) or complementary metal oxide semiconductor (CMOS) image sensors (CIS).
According to a conventional method, a photodiode is formed in a semiconductor substrate through ion implantation. However, as the size of the photodiode is gradually reduced to increase the number of pixels without increasing the chip size, a proportion of the light receiving area (that is, a fill factor) is decreased, which may result in deterioration of the image quality.
Furthermore, an airy disk, which is a phenomenon caused by diffraction of light, may occur when a stack height is not reduced as much as the decreased light receiving area. Accordingly, the number of photons incident to the light receiving parts tends to be reduced.
In order to overcome these problems, the material for the photodiode may be vapor-deposited using amorphous silicon (Si). Alternatively, the readout circuitry may be formed on a Si semiconductor wafer, while the photodiode is formed on a separate wafer, which is bonded to the readout circuitry wafer by wafer-to-wafer bonding at an upper part of the readout circuitry. Such a structure will be referred to herein as a 3D image sensor. The photodiode and the readout circuitry are connected through a metal line.
According to the conventional structure, a short may occur in the photodiode due to a contact plug connecting the readout circuitry with the photodiode.
Additionally, in the conventional structure, charge sharing may be caused since a source and a drain at terminals of a transfer transistor are both heavily doped (e.g., with N-type dopants). Occurrences of charge sharing can deteriorate sensitivity of output images or even generate image errors.
Moreover, photocharges may not be smoothly transferred from the photodiode to the readout circuitry in the conventional structure, thereby causing a dark current, or deteriorating saturation and sensitivity.